Salt removal and transport system and method for use in a mono ethylene glycol reclamation process

ABSTRACT

A system for removing salt from a process stream being fed into a flash separator introduces a swirling motive fluid into the fluid column of the separator. The swirling motive fluid comes into contact with salt components residing in the fluid of the column to create a salt slurry. Means to limit the upward movement of the swirling motive fluid can be provided. The salt slurry is then removed and can be sent to a brine generation vessel. Saturated brine from the vessel is transported back to the fluid column to replace the motive fluid in the column.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application which claims priority toU.S. patent application Ser. No. 14/307,232 filed on Jun. 17, 2014,which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

This invention relates to processes designed to treat mono ethyleneglycol (MEG) used in the oil and gas industry, especially in offshorelocations, to control hydrates formation. More particularly, theinvention relates to MEG reclamation processes which are designed toremove salts and other contaminants from a wet MEG feed stream.

In the oil and gas industry, dry (lean) MEG is used to control theformation of hydrates within the produced stream. The now wet (rich) MEGis, in turn, dried by way of a MEG reclamation process so the MEG can beused again in hydrate control.

The unit used to recover MEG usually includes three sections:pre-treatment, flash separation, and MEG regeneration. Those sectionscan be followed by salt management and calcium removal sections.

In the pre-treatment stage, the rich MEG containing some dissolved gasand hydrocarbon liquids must pass through a three-phase separatorvessel. The gas is flashed and recovered hydrocarbon liquids are sent tothe production separator. The rich MEG is sent to a flash separator. Therich MEG stream comprised of produced water and MEG is fed to the flashseparator where it is brought into contact with a hot recycle stream ofMEG. The flash separator operates under vacuum. The MEG and watercomponents of the rich MEG stream are flashed and exit through the topof the flash separator where they are sent to the MEG distillationcolumn for regeneration. The salt components of the rich MEG streamprecipitate in the flash separator.

The MEG regeneration section is a refluxed distillation column. Thedistillation column also operates under vacuum and distills the waterfrom the MEG-water vapors coming off the top of the flash separator.Salt-free, lean MEG produced at the bottom of the distillation column ispumped to storage for reuse. The vaporized water passes overhead fromthe distillation column. The water is condensed and collected in thereflux drum. A small amount is returned to the distillation column asreflux, and the remaining is routed to treatment.

The salt crystals that precipitate in the flash separator are separatedby gravity to the bottom of the brine column, where they are transferredto the salt tank. There, the salts are concentrated before removalthrough a centrifuge.

The salts in produced water cover a variety of species, but generallyare categorized into monovalent salts (typically sodium and potassium),and divalent salts (typically calcium and magnesium). The divalent saltscannot be effectively precipitated in the same manner as the monovalentsalts, so a separate calcium removal process may be installed. Effectivecalcium control is accomplished as the divalent salts are collected,reacted and removed through a centrifuge with the centrate overflowreturning to the process.

Current methods of removing the salt crystals from bottom of the brinecolumn involves a lot of equipment, including but not limited tocomplicated and expensive centrifugal filters or desanding cyclones,centrifuge pump filtration systems, a salt tank, a centrate tank, and adensity measurement device. Reducing the footprint of the systemrequired to remove the salt crystals is important for making moreefficient use of space, reducing off-shore construction costs, andincreasing ease of system operation and maintenance.

SUMMARY OF THE INVENTION

A system for removing salt from a rich mono ethylene glycol (“MEG”)stream being fed into a flash separator includes means for isolating orlimiting an upward movement of the fluid residing in the brine (fluid)or downcomer column of the separator from the MEG, a solids fluidizationdevice having means for causing a motive fluid to swirl when exiting thedevice; means for removing a salt slurry created by the swirling motivefluid.

The isolating means is preferably an on-off valve located between a topand a bottom end of the brine column of the flash separator, the on-offvalve when in the off position isolating the fluid residing in the brinecolumn above the valve from the fluid residing and below the valve. Abutterfly-type valve is an adequate on-off valve.

The solids fluidization device is located within the brine column andbelow the on-off valve and has means that cause the motive fluidentering the device to swirl when exiting the device. The swirlingmotive fluid then comes into contact with salt components residing inthe resident fluid isolated below the on-off valve. The motive fluid canbe a produced water stream, a condensate water stream, or a stream thatis some combination of the two. The swirling flow (e.g., vertiginous,rotary, or cyclonic) creates a salt slurry which is removed from thebrine column by the removal means and passed to a brine generationvessel.

The causing means is preferably a device having a plurality of slotslocated toward an upper end of the solids fluidization device andarranged tangential to, surrounding, and in communication with a centralinner bore of the device. The removal means is preferably a slurrydelivery head located in the brine column and directly above the solidsfluidization device.

The system further includes a brine generation vessel arranged toreceive the salt slurry from the brine column. The brine generationvessel provides a saturated brine which can be transported back into thebrine column and replace the motive fluid which was introduced earlier.

A method of removing salt from a rich MEG stream being fed into a flashseparator includes the steps of:

-   i. isolating fluid containing solid/salt already residing toward a    lower end of the brine column from MEG residing toward an upper end    of the brine column;-   ii. introducing a swirling motive fluid stream into a bottom end of    the brine column to form a salt slurry stream; and-   iii. removing the salt slurry stream.

The isolating step is preferably done by way of an on-off valve, whichcould be a butterfly-type valve. The introducing step is preferably byway of a solids fluidization device having a plurality of spaced-apartvertical slots arranged tangential to, surrounding, and in communicationwith a central inner bore of the device. The removal step is by way of aslurry delivery head located above the solids fluidization device. Afterthe step iii, a saturated brine stream can be introduced into the brinecolumn as a way to displace the motive fluid. After the saturated brinestream is introduced into the column, the valve can be opened and theisolated fluids can be allowed to come into contact with one anotheragain.

The objectives of this invention are to (1) eliminate the need forcomplicated and expensive centrifugal filters and desandinghydrocyclones to remove salt; (2) eliminate the need for centrifugefiltration, a salt tank, a centrate tank, and density measurementdevices; and (3) require less foot print than the prior art systems andmethods and have lower construction costs and be easier to operate andmaintain than those prior art systems and methods.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a preferred embodiment of a salt transportsystem for a MEG reclamation or recovery process. The system includes asolids fluidization device located at the bottom end of the brine columnof a flash separator and an on-off valve located between thefluidization device and the MEG/brine transition zone of the flashseparator.

FIG. 2 is a front elevation view of a preferred embodiment of the solidsfluidization device of FIG. 1.

FIG. 3 is top view of the solids fluidization device of FIG. 2.

FIG. 4 is a cross-section view of the solids fluidization device of FIG.2.

FIG. 5 is a cross-section view of the solids fluidization device of FIG.2 taken along section line 5-5 of FIG. 2.

FIG. 6 is an enlarged view of the solids fluidization device of FIG. 1and the removal device located directly above the solids fluidizationdevice.

ELEMENTS AND NUMBERING USED IN THE DRAWINGS

10 Salt transport system 20 Flash separator 21 Upper end 25 Rich (wet)MEG stream 27 Water and MEG vapor stream 29 Brine (fluid) or downcomercolumn or section 31 MEG/brine transition zone 33 On-off valve 35Recycle loop 37 Bottom or lower end of 29 39 Upper end of 29 40 Sandremoval device 41 Inlet 43 Upper end of 40 45 Slots 47 Inner bore 49Produced or condensate (carrier or motive) water stream 51 Swirlingmotive fluid stream 53 Salt slurry stream 55 Removal device or slurrydischarge head 60 Brine generation vessel 61 Agitator 63 Brine stream 65Salt slurry (discharge) stream

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring first to FIGS. 1 and 6, a preferred embodiment of a salttransport system 10 for a MEG recovery or reclamation process includes aflash separator 20 having a solids fluidization device 40 located at thebottom end 37 of the brine or downcomer column 29 and an on-off valve 33located between the device 40 and the MEG/brine transition zone 31.

The flash separator 20 is of a kind well known in the art. In theseparator 20 a rich (wet) MEG inlet stream 25 is brought into contactwith a hot MEG recycle stream 35. The MEG and water components of therich MEG stream 25 are flashed and exit the upper end 21 of theseparator 20 as a water and MEG vapor stream 27. The salt components ofthe rich MEG inlet stream 25 precipitate in the brine (fluid) column 29of the separator 20. A MEG/brine transition zone 31 forms in the column29 between the MEG and the brine.

Solids fluidization device 40 is arranged at the bottom end 37 of thecolumn 29. The device 40 includes means which produce or cause aswirling (e.g. vertiginous, rotary or cyclonic) motion or flow 51 of themotive fluid as it exits device 40. One suitable device 40 is aHYDROTRANS™ solids fluidization and removal device (Cameron ProcessSystems, Houston, Tex.). Any other device may be used as thefluidization device provided the device creates a swirling (e.g.,vertiginous, rotating, or cyclonic) motive fluid flow when the flowexits the device.

Referring to FIGS. 2-5, the HYDROTRANS™ device includes a plurality ofspaced-apart slots 45 arranged tangential to, surrounding, and incommunication with an inner bore 47 which receives a motive fluid stream49 at the lower inlet end 41 of the device. Motive fluid steam 49—whichcan be a produced water or condensate water stream (or some combinationthereof)—exits the slots 45 of device 40 as a swirling motive fluidstream 51. The swirling motion of the motive fluid stream 51 mixes withthe fluid containing solid/salt already residing in the column 29 tofluidize the salt components, thereby creating a salt slurry stream 53.By way of example, during the first five minutes of operation, theconcentration of salt in the device 40 can be about 20 vol % on average.

Unlike a desanding hydrocyclone—which produce a cyclonic flow within thedevice but a straight over- and underflow exiting the device (i.e.,straight in, cyclonic within, and straight out)—the solids fluidizationdevice 40 produces this type of flow external to the device (i.e.,straight in and rotary or cyclonic out).

The removal device 55, which can be a slurry discharge head, residesjust above the upper end 43 of solids fluidization device 40. Removaldevice 55 carries the salt slurry stream 53 to a brine generation tankor vessel 60.

Because the brine in the column 29 is saturated with salt, addingproduced water to it causes the lower density (not saturated) producedwater 49 to flow to the upper end 39 of the column 29 and MEG to flow tothe bottom end 37. This causes MEG loss. To prevent this loss fromoccurring, system 10 limits upward movement of the fluid, which can beby way of isolation means such as on-off valve 33, which may be abutterfly-type valve. When the valve 33 is in the off or closedposition, it prevents the produced water from flowing to the upper end39 of the column 29. The valve 33 isolates the fluid or brine locatedabove and below the valve 33 from one another.

Once the salts are removed from the bottom end 37 of the column 29, thesaturated brine in the brine generation vessel 60 is pumped back to thecolumn 29 below the on-off valve 33 to replace the produced water. Oncethe produced water is replaced with the saturated brine, the on-offvalve 33 is put in the on or open position to allow the salt to settlebelow the valve 33 and into the bottom 37 of column 29.

If a 1″ HYDROTRANS is used as device 40, a flow rate of 4 m³/hr isrequired to remove the salt from the bottom end 37. For about the firstfive minutes of operation, about 0.33 m³ of a salt slurry stream 53(about 20 vol %) to the brine generation vessel 60. Assuming a voidspace of 40% between the salt particles (i.e., the salt represents 60%),the total amount of salt removed in five minutes (0.083 hr) is 0.04 m³(4 m³/hr×0.083 hr×0.2×0.6). The salt density is 2,165 kg/m³. Therefore,the amount of salt removed in five minutes of operation (i.e., with thevalve 33 closed) is about 87 kg. If the amount of salt settled at thebottom end 37 is higher (or lower) than in the example, the removalprocess can be adjusted accordingly.

System 10 does not require any centrifugal filters or desanding cyclonesto remove salt from the brine column 29, nor does it require centrifugalfiltration, salt and centrate tanks, and density measurement devices.System 10 also requires less foot print than the prior art systems andmethods, has lower construction costs, and is easier to operate andmaintain.

After the salt removal process is completed, an agitator 61 can be usedto agitate and dissolve the salt in the liquid phase within the brinegeneration vessel 60. The saturated brine solution can then be pumped asa saturated brine stream 63 to the column 29 to replace the producedwater. Once this operation is complete, the valve 33 can be put in theon or open position to accumulate salt in the bottom end 37 of thecolumn 29.

When the brine generation vessel 60 is filled with enough salt, agitator60 will again be turned on to make a salt slurry stream 65 which ispumped to a water treating unit (not shown) or to overboard (ifallowed).

A method of removing salt from a rich MEG stream which makes use ofsystem 10 includes the steps of:

-   -   i. isolating fluid in the brine column 29 of the flash separator        20 by closing a valve 33 located above the bottom end 37 of the        brine column 29 and below the upper end 39 of the column 29;    -   ii. introducing a swirling motive fluid stream 51 into the        bottom end 37 of the brine column 29, the swirling motive fluid        stream 51 coming into contact with salt components residing in        the column 29 and forming a salt slurry stream 53;    -   iii. removing the salt slurry stream 53 from the brine column 29        to a brine generation vessel 60;    -   iv. agitating the contents of the brine generation vessel 60 to        form a saturated brine 63;    -   v. transferring the saturated brine 63 back to the column 29;        and    -   vi. opening the valve 33 after step v is completed.

Salt removal system 10 and the method for its use is an improvement overprior art systems and methods. The prior art makes use of complicatedand expensive centrifugal filters or desanding cyclones to remove saltfrom the brine column 29 of the flash separator 20 along with centrifugefiltration, a salt tank, a centrate tank, and density measurementdevices.

While preferred embodiments of system 10 and a method for its use havebeen described in detail, the scope of the invention is defined by thefollowing claims.

What is claimed:
 1. A method of fluidizing salts to aid salts removal,the method comprising the steps of: i. introducing a swirling motivefluid into a bottom end of a fluid column of a flash separator, theswirling motive fluid contacting salt components of a fluid residing inthe fluid column; ii. limiting an upward movement of the swirling motivefluid within the fluid column; and iii. removing a salt slurry producedby the swirling motive fluid.
 2. A method according to claim 1 whereinthe limiting step is by way of a valve located in the fluid column.
 3. Amethod according to claim 1 further comprising the step of recirculatingthe salt slurry to the fluid column.
 4. A method according to claim 1further comprising the step of introducing, after the step ii, asaturated brine stream into the fluid column.
 5. A method according toclaim 1 wherein a source of the swirling motive fluid is at least one ofa produced water stream and a condensate water stream.